The separation of U from Np and Pu are major stages in the Purex process. Simple hydroxamic acids (XHA) are salt free, hydrophilic organic compounds, RCONHOH, with affinities for cations such as Fe3+, Np4+ and Pu4+. They are also redox active, capable of reducing a range of metal ions e.g. Np(VI) to Np(V). These two properties have led to them being identified as useful reagents for the control of Pu and Np in an Advanced Purex process. The kinetics of the acid hydrolysis of free formo- (FHA) and acetohydroxamic (AHA) acids to hydroxylamine and the parent carboxylic acid are well known. Hydrolysis of hydroxamates bound to metal ions also occurs and preliminary studies have shown that the Pu(IV)-FHA and AHA complexes are slowly reduced to free Pu(III) ions. An understanding of these processes is vital if they are to be controlled within the design of an Advanced Purex process. To this end, we have used UV-visible & nIR spectrophotometry to study and theoretically model the kinetics of the hydrolysis of metal-HA systems in nitrate media – Fe3+ / AHA (as a non-active analogue); Np4+ / FHA, Np4+/AHA – wherein the metal ion complexes with, but does not oxidise the ligand.